Field of the Invention

The present invention relates to lithium metal phosphate materials. More specifically, the present invention relates to lithium metal phosphate materials for use in secondary lithium ion batteries.

Background of the Invention

Lithium metal phosphates with olivine structures have emerged as promising cathode materials in secondary lithium ion batteries. Advantages of lithium metal phosphates compared with other lithium compounds include the fact that they are relatively benign environmentally, and have excellent safety properties during battery handling and operation.

It is desirable that lithium metal phosphate materials have high gravimetric capacity, high volumetric capacity, and high rate capability. Gravimetric and volumetric capacities can indicate the total amount of energy that a material can store, whilst rate capability will be understood to indicate how quickly that energy can be extracted from said material.

Lithium metal phosphate cathodes may be prepared from electroactive materials in the form of agglomerated particles. Cathodes comprising agglomerated particles typically show good gravimetric capacity and rate capability but relatively poor volumetric capacity.

Conversely, cathodes comprising powder lithium metal phosphate materials, which may be in the form of essentially primary particles (i.e. non-agglomerated particles) or fragments of agglomerated materials typically show reasonable volumetric capacities but poor gravimetric capacities and rate capability.

To overcome these defects, it has been suggested that agglomerated materials may be milled or sifted to produce compositions which have a mixture of agglomerated particles and smaller particles, for example primary particles or fragments of agglomerated particles. Such milled or sifted materials usually show bimodal particle size distributions.

Agglomerated lithium metal phosphates may be prepared by a hydrothermal process and a milling step such as that described in WO2014/14140323A1 which is incorporated herein in its entirety by reference. A hydrothermal process is also described in W02005/051840A1 which is incorporated herein in its entirety by reference. Powder lithium metal phosphate may be prepared according to the process described in WO2 005/051840 A 1 the contents of which are incorporated herein in their entirety by reference.

US2015072230A1 discloses a cathode material with a multimodal particle size distribution formed by combining materials with two or more different particle size distributions.

US2012156560A1 discloses a mixture of small particles and large particles of a layered lithium metal oxide material.

There remains a need for lithium metal phosphate materials having an excellent balance of properties and which combine high gravimetric capacities, high volumetric capacities, and high rate capabilities.

Summary of the Invention

The present inventors have surprisingly found that by providing specific particle compositions the electrochemical properties of lithium metal phosphates can be tuned to provide a particulate electrode material with excellent gravimetric and volumetric properties, and excellent discharge capabilities.

In a first aspect of the present invention there is provided a composition comprising carbon- coated particles of agglomerated lithium metal phosphate and carbon-coated particles of powder lithium metal phosphate, the agglomerated and the powder lithium metal phosphates each independently having general formula:

LiFei- _x M _x PO4 in which 0<x<1 and M is one or more selected from Ni, Co, Mn, Ca, Zn, Al, B, Ti and Mg, wherein the particle size distribution of the composition has: a) when measured using a Malvern MasterSizer 2000 in ethanol, three or more peaks having at least peak modes at 0.1 -0.7 m, 0.5-2 pm, and 3-20 pm, and 25 volume % of the particles having a particle size of 0.5 pm or less and 75 volume% of the particles having a particle size of 10 pm or less; and b) when measured using a Malvern MasterSizer 2000 in air at a gas pressure of 0.2 bar, a particle size distribution comprising two or more peaks having at least peak modes at 0.5-4 pm, and 5-30 pm, and 25 volume % of the particles having a particle size of 2 ,m or less and 75 volume% of the particles having a particle size of 20 ,m or less.

It has surprisingly been found that compositions of the invention are able to provide high volumetric capacities whilst simultaneously achieving high gravimetric capacities and rate capabilities when used as a cathode material in an electrode of a secondary lithium ion battery. In addition, materials of the first aspect of the invention further exhibit an excellent balance of properties. Specifically, materials of the first aspect have good processability, resistance, polarisation, and first cycle efficiencies.

Laser diffraction techniques are commonly used to determine the particle size distribution of particulate materials, for example using a Malvern MasterSizer 2000.

The particle size distribution of the composition of the invention may be analysed by suspending it in a carrier fluid such as liquid (e.g. ethanol), or a gas (e.g. air). The choice of carrier fluid has been found to influence the physical behaviour of the composition during analysis. Different apparent particle size distributions are realised depending on the choice of carrier fluid. It is thought that smaller particles (e.g. powder particles, fragments of agglomerated particles, or primary particles) of carbon-coated lithium metal phosphate agglomerate when air is the carrier fluid, whilst agglomerated particles may be broken up when ethanol is the carrier fluid. The present inventors further believe that a “perfect” carrier fluid may not exist and so taking particle size measurements in more than one carrier fluid is sometimes necessary.

Material of the prior art (e.g. compositions formed by milling and/or sifting agglomerates) sometimes appear to be comprised of a mixture of large (e.g agglomerated) particles and smaller particles (e.g. powder particles, fragments of agglomerated particles, or primary particles). These materials may exhibit a bimodal or multimodal particle size distribution when analysed using laser diffraction techniques (e.g. using a Malvern MasterSizer 2000). Particle size analysis typically show the smaller particles as peaks, or modes, between about 0.1 to 2 .m, whilst the large (e.g. agglomerated) particles may appear as peaks between about 5-30 .m. However, these materials may not in fact exist as a mixture of large (e.g. agglomerated) particles and smaller particles (e.g. powder, fragments or agglomerates, or primary particles), despite the bimodal/multimodal appearance of their particle size distributions. Without being bound to any specific theory it is believed that the smaller particles are weakly bound to, and remain associated with, the large (e.g. agglomerated) particle under most conditions. In other words, the smaller powder particles seen in the particle size distribution analysis may not actually be present as “free” particles.

Consequently, these smaller particles (e.g. powder particles, fragments of agglomerated particles, or primary particles) may not be involved in packing of the carbon-coated lithium metal phosphate during formation/compaction into an electrode.

The composition of the first aspect of the invention may be prepared as a mixture of two different carbon-coated lithium metal phosphate materials. The composition of the first aspect comprises carbon-coated particles of an agglomerated lithium metal phosphate and carbon-coated particles of a powder lithium metal phosphate. The carbon-coated particles of the agglomerated lithium metal phosphate typically have a multimodal particle size distribution when measured in air at a pressure of 0.2 bar using a Malvern MasterSizer 2000. The carbon-coated particles of the powder lithium metal phosphate may be in the form of essentially non-agglomerated primary particles or fragments of agglomerates wherein 50 volume percent of the particles typically have a particle size of 0.7 .m or less when measured in ethanol using a Malvern MasterSizer 2000. The weight ratio of the carbon- coated particles of the agglomerated lithium metal phosphate to the carbon-coated particles of the powder lithium metal phosphate is typically from 1.85-3:1.

Accordingly, in a further aspect of the invention there is provided a composition comprising carbon-coated particles of an agglomerated lithium metal phosphate and carbon-coated particles of a powder lithium metal phosphate, the agglomerated and the powder lithium metal phosphate each independently having general formula:

LiFei- _x M _x PO4 in which 0<x<1 and M is one or more selected from Ni, Co, Mn, Ca, Zn, Al, B, Ti and Mg, wherein the-carbon-coated particles of the agglomerated lithium metal phosphate have a multimodal particle size distribution when measured in air at a pressure of 0.2 bar using a Malvern MasterSizer 2000, and the carbon-coated particles of the powder lithium metal phosphate are in the form of essentially non-agglomerated primary particles or fragments of agglomerates wherein 50 volume percent of the particles have a particle size of 0.7 .m or less when measured in ethanol using a Malvern MasterSizer 2000, and wherein the weight ratio of the carbon-coated particles of the agglomerated lithium metal phosphate to the carbon-coated particles of the powder lithium metal phosphate is from 1.85-3:1. According to a further aspect of the invention there is provided a process for preparing a composition according to the invention.

In a further aspect of the invention there is provided a cathode comprising a composition of the invention.

In a further aspect of the invention there is provided a secondary lithium ion battery comprising a cathode of the invention.

In a further aspect of the invention there is provided a lithium metal phosphate composition obtained or obtainable by a process of the invention.

Brief Description of the Drawings

Figure 1 shows the volume based particle size distribution of a composition according to the invention as determined using a Malvern MasterSizer 2000 in air at a pressure of 0.2 bar.

Figure 2 shows the volume based particle size distribution of a composition according to the invention as determined using a Malvern MasterSizer 2000 in ethanol.

Figure 3 shows the volume based particle size distribution of carbon-coated particles of an agglomerated lithium iron phosphate (P2S2) available from Johnson Matthey, determined using a Malvern MasterSizer 2000 in air at a pressure of 0.2 bar.

Figure 4 shows the volume based particle size distribution of carbon-coated particles of powder lithium iron phosphate (P2), available from Johnson Matthey, as determined using a Malvern MasterSizer 2000 in ethanol.

Figure 5 shows the volume based particle size distribution of carbon-coated particles of an agglomerated lithium iron phosphate (P2S2) available from Johnson Matthey, determined using a Malvern MasterSizer 2000 in ethanol.

Figure 6 shows the volume based particle size distribution of carbon-coated particles of a powder lithium iron phosphate (P2), available from Johnson Matthey, as determined using a Malvern MasterSizer 2000 measured in air at a pressure of 0.2 bar. Figure 7 shows the volume based particle size distribution of carbon-coated particles of an agglomerated lithium iron phosphate determined using a Malvern MasterSizer 2000 measured in air at a pressure of 0.2 bar following a sieving treatment.

Figure 8 shows the volume based particle size distribution of carbon-coated particles of an agglomerated lithium iron phosphate determined using a Malvern MasterSizer 2000 measured in air at a pressure of 0.2 bar following a sifting treatment.

Detailed Description

Preferred and/or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention unless the context demands otherwise. Any of the preferred and/or optional features of any aspect may be combined, either singly or in combination, with any aspect of the invention unless the context demands otherwise.

The present invention provides a composition comprising carbon-coated particles of an agglomerated lithium metal phosphate and carbon-coated particles of a powder lithium metal phosphate.

The carbon-coated particles of the agglomerated and the powder lithium metal phosphate each independently have general formula LiFei. _x M _x PO4. M may be one or more elements selected from the group comprising Ni, Co, Mn, Ca, Zn, Al, B, Ti and Mg. Suitably, M may be one or more elements selected from the group comprising Mn, Ni, Al, and Co. Preferably, M may be Al and/or Mn.

In the general formula LiFei- _x M _x PC>4, x may be greater than or equal to 0, greater than or equal to 0.05, greater than or equal to 0.1 , greater than or equal to 0.2, greater than or equal to 0.3. x may be less than or equal to 0.9, less than or equal to 0.7, less than or equal to 0.5, or less than or equal to 0.3. Typically, x may be greater than or equal to 0 and less than or equal to 0.9, greater than or equal to 0.05 and less than or equal to 0.7, greater than or equal to 0.1 and less than or equal to 0.5, or greater than or equal to 0.2 and less than or equal to 0.3. In some embodiments the agglomerated and/or the powder lithium metal phosphate both have general formula LiFePC In other words, x is essentially 0 and any inclusion of M is due to incidental impurities, for example impurities in the reagents used or from the manufacturing equipment used. It will be understood that the general formula of the agglomerated and powder lithium metal phosphates may be the same or different. Typically, the general formula of the agglomerated and powder lithium metal phosphate is the same.

The carbon-coated particles of the agglomerated lithium metal phosphate may comprise secondary agglomerates of primary particles. The carbon-coated particles of the agglomerated lithium metal phosphate may be formed of a plurality of smaller particles, which may be, for example, primary particles or fragments of agglomerates.

The carbon-coated particles of the powder lithium metal phosphate may be present in the form of essentially primary particle and/or fragments of agglomerated particles.

Primary particles will be understood to refer to discrete particles which are themselves not formed of agglomerates or aggregates of other particles. Fragments of agglomerates will be understood to include agglomerated particles which have fragmented into smaller particles. Primary particles and/or fragments of agglomerates typically have a particle size of from 0.1 to 2 .m.

The composition typically has a particle size distribution which is defined with respect to its particle size distribution as measured using a Malvern MasterSizer 2000 in two separate carrier fluids; a) in ethanol, and b) in air at a gas pressure of 0.2 bar.

As will be understood by those skilled in the art that peak modes, as used herein, may also be referred to as peak maxima.

For the avoidance of doubt values of particle size given as 25 volume percent or less, 50 volume percent or less, and 75 volume percent or less correspond to a particle size value where 25 volume percent, 50 volume percent, and 75 volume percent have a value of or below the given particle size. Said values may be calculated by integrating the area under the particle size distribution plot.

The start and end points from peak modes defined below, for particle size distribution measured in ethanol, may be combined in any order to form a new range.

The particle size distribution of the composition as measured using a Malvern MasterSizer 2000 in ethanol typically comprises three or more peaks. The peak modes of the three or more peaks are present at from 0.1 to 0.7 .m, from 0.5 to 2 .m, and from 3 to 20 .m. The peak modes of the three or more peaks may be present at from 0.13 to 0.6 pm, from 0.6 to 1.8 pm, and from 4 to 18 pm. The peak modes of the three or more peaks may be present at from 0.15 to 0.5 pm, from 0.7 to 1.6 pm, and from 5 to 16 pm. The peak modes of the three or more peaks may be present at from 0.2 to 0.4 pm, from 0.8 to 1.5 pm, and from 6 to 15 pm.

25 volume percent of the particles of the composition typically have a particle size of 0.5 pm or less when the particle size distribution of the composition is measured using a Malvern MasterSizer 2000 in ethanol. For example, 25 volume percent of the particles of the composition may have a particle size of 0.45 pm or less, 0.4 pm or less, or 0.35 pm or less.

50 volume percent of the particles of the composition may have a particle size of 1 pm or less when the particle size distribution of the composition is measured using a Malvern MasterSizer 2000 in ethanol. For example, 50 volume percent of the particles of the composition may have a particle size of 0.9 pm or less, 0.8 pm or less, or 0.7 pm or less.

75 volume percent of the particles of the composition typically have a particle size of 10 pm or less when the particle size distribution of the composition is measured using a Malvern MasterSizer 2000 in ethanol. For example, 75 volume percent of the particles of the composition may have a particle size of 9 pm or less, 8 pm or less, or 7 pm or less.

The start and end points from peak modes defined below, for particle size distributions measured in air at a pressure of 0.2 bar, may be combined in any order to form a new range.

The particle size distribution of the composition as measured using a Malvern MasterSizer 2000 in air at a gas pressure of 0.2 bar typically comprises two or more peaks. The peak modes of the two or more peaks are present at from 0.5 to 4 pm, and 5 to 30 pm. The peak modes of the two or more peaks may be present at from 0.7 to 3 pm, and 8 to 25 pm. The peak modes of the two or more peaks may be present at from 0.8 to 2.5 pm, and 10 to 22 pm. The peak modes of the two or more peaks may be present at from 1 to 2 pm, and 12 to 20 pm.

25 volume percent of the particles of the composition typically have a particle size of 2 pm or less when the particle size distribution of the composition is measured using a Malvern MasterSizer 2000 in air at a pressure of 0.2 bar. For example, 25 volume percent of the particles of the composition have a particle size of 1.9 .m or less, 1.8 .m or less, or 1.7 pm or less.

50 volume percent of the particles of the composition typically have a particle size of 15 pm or less when the particle size distribution of the composition is measured using a Malvern MasterSizer 2000 in air at a pressure of 0.2 bar. For example, 50 volume percent of the particles of the composition may have a particle size of 14 pm or less, 13 pm or less, or 12 pm or less.

75 volume percent of the particles of the composition typically have a particle size of 20 pm or less when the particle size distribution of the composition is measured using a Malvern MasterSizer 2000 in air at a pressure of 0.2 bar. For example, 75 volume percent of the particles of the composition have a particle size of 19 pm or less, 18 pm or less, or 17 pm or less.

The BET surface area of the composition may be 7 m ² /g or more, 8 m ² /g or more, 9 m ² /g or more, or 10 m ² /g or more. The carbon-coated particles of the powder lithium metal phosphate may have a BET surface area of 18 m ² /g or less, 16 m ² /g or less, 15 m ² /g or less, or 14 m ² /g or less. For example, the BET surface area of the composition may be from 7 to 18 m ² /g, from 8 to 16 m ² /g, from 9 to 15 m ² /g, or from 10 to 14 m ² /g.

The agglomerated lithium metal phosphate and powder lithium metal phosphate comprise an electrically conductive carbon coating on at least a part of their surface. The weight percentage of carbon present relative to the total amount of agglomerated lithium metal phosphate or powder lithium metal phosphate is not particularly limited but is typically between 0.5 and 4 wt%.

The carbon-coated particles of the agglomerated lithium metal phosphate typically have a multimodal particle size distribution when measured in air at a pressure of 0.2 bar using a Malvern MasterSizer 2000.

The multimodal particle size distribution of the carbon-coated particles of the agglomerated lithium metal phosphate as measured using a Malvern MasterSizer 2000 in air at a gas pressure of 0.2 bar may comprise two of more peaks. The peak modes of the two or more peaks may be present at from 0.5 to 4 .m, and 4 to 25 .m. The peak modes of the two or more peaks may be present at from 0.6 to 3 .m, and 6 to 20 .m. The peak modes of the two or more peaks may be present at from 0.7 to 2 pm, and 10 to 18 pm. The peak modes of the two or more peaks may be present at from 0.8 to 1.5 pm, and 12 to 16 pm.

25 volume percent of the carbon-coated particles of the agglomerated lithium metal phosphate may have a particle size of 3 pm or less when the particle size distribution is measured using a Malvern MasterSizer 2000 in air at a pressure of 0.2 bar. For example, 25 volume percent of the carbon-coated particles of the agglomerated lithium metal phosphate may have a particle size of 2.7 pm or less, 2.5 pm or less, or 2 pm or less.

50 volume percent of the carbon-coated particles of the agglomerated lithium metal phosphate may have a particle size of 15 pm or less when the particle size distribution is measured using a Malvern MasterSizer 2000 in air at a pressure of 0.2 bar. For example, 50 volume percent of the carbon-coated particles of the agglomerated lithium metal phosphate may have a particle size of 14 pm or less, 13 pm or less, or 12 pm or less.

75 volume percent of the carbon-coated particles of the agglomerated lithium metal phosphate may have a particle size of 30 pm or less when the particle size distribution is measured using a Malvern MasterSizer 2000 in air at a pressure of 0.2 bar. For example, 75 volume percent of the carbon-coated particles of the agglomerated lithium metal phosphate may have a particle size of 25 pm or less, 22 pm or less, or 20 pm or less.

The carbon-coated particles of the powder lithium metal phosphate may have a multimodal particle size distribution when measured in ethanol using a Malvern MasterSizer 2000.

The multimodal particle size distribution of the carbon-coated particles of the powder lithium metal phosphate as measured using a Malvern MasterSizer 2000 in ethanol may comprise two or more peaks. The peak modes of the two or more peaks may be present at from 0.1 to 0.7 pm, and 0.8 to 3 pm. The peak modes of the two or more peaks may be present at from 0.15 to 0.6 pm, and 0.9 to 2.5 pm. The peak modes of the two or more peaks may be present at from 0.18 to 0.5 pm, and 1 to 2.2 pm. The peak modes of the two or more peaks may be present at from 0.2 to 0.4 pm, and 1.1 to 2 pm.

25 volume percent of the carbon-coated particles of the powder lithium metal phosphate may have a particle size of 0.5 pm or less when the particle size distribution is measured using a Malvern MasterSizer 2000 in ethanol. For example, 25 volume percent of the carbon-coated particles of the powder lithium metal phosphate may have a particle size of 0.4 .m or less, 0.35 .m or less, or 0.3 pm or less.

50 volume percent of the carbon-coated particles of the powder lithium metal phosphate may have a particle size of 0.7 pm or less when the particle size distribution is measured using a Malvern MasterSizer 2000 in ethanol. For example, 50 volume percent of the carbon-coated particles of the powder lithium metal phosphate may have a particle size of 0.6 pm or less, 0.55 pm or less, or 0.5 pm or less.

75 volume percent of the carbon-coated particles of the powder lithium metal phosphate may have a particle size of 1.5 pm or less when the particle size distribution is measured using a Malvern MasterSizer 2000 in ethanol. For example, 75 volume percent of the carbon-coated particles of the powder lithium metal phosphate may have a particle size of 1.4 pm or less, 1 .3 pm or less, or 1.2 pm or less.

The weight ratio of the carbon-coated particles of the agglomerated lithium metal phosphate to the carbon-coated particles of the powder lithium metal phosphate in the composition is 1.85-3:1. For example, the weight ratio may be 1.9-2.8:1 , the weight ratio may be 1.8-2.6:1 , the weight ratio may be 1.7-2.4:1. In certain preferred embodiments the weight ratio of the carbon-coated particles of the agglomerated lithium metal phosphate to the carbon-coated particles of the powder lithium metal phosphate is 2.33:1.

The carbon-coated particles of the agglomerated lithium metal phosphate may have a BET surface area of 7 m ² /g or more, 8 m ² /g or more, 9 m ² /g or more, or 9.5 m ² /g or more. The carbon-coated particles of the agglomerated lithium metal phosphate may have a BET surface area of 30m ² /g or less, 28 m ² /g or less, 27 m ² /g or less, 26 m ² /g or less, 25 m ² /g or less, or 24 m ² /g or less. For example, the carbon-coated particles of the agglomerated lithium metal phosphate may have a BET surface area of 7 to 30 m ² /g, 8 to 28 m ² /g, 9 to 27 m ² /g, or 9.5 to 26 m ² /g.

The carbon-coated particles of the powder lithium metal phosphate may have a BET surface area of 10 m ² /g or more, 11 m ² /g or more, 12 m ² /g or more, or 14 m ² /g or more. The carbon- coated particles of the powder lithium metal phosphate may have a BET surface area of 32 m ² /g or less, 31 m ² /g or less, 30 m ² /g or less, or 29 m ² /g or less. For example, the carbon- coated particles of the powder lithium metal phosphate may have a BET surface area of from 10 to 32 m ² /g, from 11 to 31 m ² /g, from 12 to 30 m ² /g, or from 14 to 29 m ² /g. The carbon-coated particles of the agglomerated lithium metal phosphate and the carbon- coated particles of the powder lithium metal phosphate are typically prepared by a hydrothermal process. Such a method involves the combination of an iron (II) source with at least one lithium source, at least one phosphate source, and optionally at least one source of M, and obtaining particulate lithium metal phosphate under hydrothermal conditions.

Suitable iron (II) sources include iron sulphate (FeSCL), typically in the form of a hydrate, and iron oxalate.

Suitable lithium sources include lithium carbonate (U2CO3), lithium hydrogen phosphate (U2HPO4), lithium hydroxide (LiOH), lithium fluoride (LiF), lithium chloride (LiCI), lithium bromide (LiBr), lithium iodide (Lil), lithium phosphate (U2PO4) or mixtures thereof. Lithium hydroxide may be preferred.

Suitable phosphate sources include phosphoric acid, metaphosphoric acid, pyro-phosphoric acid, triphosphoric acid, tetraphosphoric acid, hydrogen phosphates or dihydrogen phosphates, such as ammonium phosphate or ammonium dihydrogen phosphate, lithium phosphate or iron phosphate or any desired mixtures thereof. Phosphoric acid is particularly preferred.

Suitable sources of M, if applicable, include sulphates and I or oxides of M or mixtures thereof. It will be understood by the skilled person that M may also be present in the iron (II) source, the lithium source, or the phosphate source, and therefore an additional source of M may not need to be added to achieve the desired level of M in the lithium metal phosphate.

Where M comprises aluminium, suitable aluminium sources include aluminium hydroxide (AI(OH)3), aluminium chloride (AICI3), aluminium sulphate (Ah(SO4)3*xH2O (typically 0 < x < 18)) , and aluminium oxide (AI2O3). Aluminium hydroxide or aluminium sulphate may be particularly preferred.

Where M comprises manganese, suitable manganese sources include manganese nitrate (Mn(NC>3)2), manganese carbonate (Mn(CC>3)2), and manganese sulphate (MnSCL).

In the context of the present invention, the term hydrothermal conditions is to be understood to refer to treatment of the precursor mixture at a temperature above room temperature and a steam pressure of above 1 bar. The hydrothermal treatment can be carried out in a manner known to the person skilled in the art, for example as described in W02005/051840 the content of which is hereby incorporated by reference. It is preferable for the hydrothermal treatment to be carried out at temperatures of between 100 to 250° C, in particular from 100 to 180° C and at a steam pressure of from 1 bar to 40 bar, in particular at a steam pressure from 1 bar to 10 bar. The precursor mixture is typically reacted in a tightly closed or pressure-resistant vessel. The reaction preferably takes place in an inert or protective gas atmosphere. Examples of suitable inert gases include nitrogen, argon, carbon dioxide, carbon monoxide or mixtures thereof. The hydrothermal treatment may, for example, be carried out for 0.5 to 15 hours, in particular for 6 to 11 hours. Purely as a non-limiting example, the following specific conditions may be selected: 1.5 hour heat-up time from 50° C (temperature of the precursor mixture) to 160° C, 10 hour hydrothermal treatment at 160° C, 3 hours cooling from 160° C to 30° C

The particles of the agglomerated and the particles of the powder lithium metal phosphate are carbon-coated. In order to form the carbon coating, the lithium metal phosphate formed by the hydrothermal process is typically mixed with a carbon source and then spray dried prior to a heating, or calcination step.

The nature of the carbon source is not particularly limited in the present invention. The carbon source is typically a carbon-containing compound which decomposes to a carbonaceous residue when exposed to the calcination step. For example, the carbon source may be one or more of starch, maltodextrin, gelatine, polyol, sugar (such as mannose, fructose, sucrose, lactose, glucose, galactose), and carbon-based polymers such as polyacrylate, polyvinyl acetate (PVA), glucono delta-lactone (GDL), and polyvinyl butyrate (PVB). Alternatively, the carbon source may be elemental carbon, such as one or more of graphite, carbon black, acetylene black, carbon nanotubes and carbon fibres (such as vapour grown carbon fibres, VGCF). Lactose or maltodextrin may be particularly preferred.

The amount of carbon source added is not particularly limited in the present invention. For example, the amount of carbon source added may be selected to yield particles of the agglomerated or the powder lithium metal phosphate with a carbon content of 1 to 5 wt%, for example 1.5 to 3.5 wt%. The amount of carbon source added may be in the range from 7 to 22 wt% based on the weight of the particulate lithium metal phosphate, for example from 10 to 18 wt%, depending on the nature of the carbon precursor, and its carbonisation yield.

The skilled person will understand that the carbon source may be combined with the lithium metal phosphate by any one of a number of means. For example, the lithium metal phosphate may be mixed with the carbon source in the presence of a solvent, such as water, and the mixture then spray dried. It will also be understood by the skilled person that in some cases it may be preferable that the carbon source is added to the precursor mixture prior to hydrothermal treatment. In such a case, it will be understood that the addition of a carbon source after hydrothermal treatment may be no longer required.

The process to produce the lithium metal phosphate suitably comprises a heating step. The lithium metal phosphate and carbon source are heated to provide the carbon-coated particles of agglomerated lithium metal phosphate. The heating step performs two functions. Firstly, it results in pyrolysis of the carbon source to form a conductive carbon coating on the lithium metal phosphate particles. Secondly, to improve the crystallinity and/or to heal potential defects of the lithium metal phosphate crystals.

Typically, the heating step is carried out in an inert atmosphere, for example in an inert gas such as argon. It may alternatively be carried out in a reducing atmosphere. It is typically carried out at a temperature in the range from 550°C to 800 °C, e.g. from 630 °C to 780 °C, or from 650 °C or 700 °C to 780 °C. 750 °C is particularly suitable. Typically, the calcination is carried out for a period of 0.4 to 10 hours. The heating time depends on the scale of manufacture (i.e. where larger quantities are prepared, longer heating times may be preferred). At a commercial scale, 0.5 to 3 hours may be suitable, for example.

Following the heating step the carbon-coated lithium metal phosphate may be subjected to a milling and / or a sifting step to provide a material with the desired multimodal particle size distribution.

The particles of the agglomerated lithium metal phosphate are preferably prepared by sifting the material produced in the heating step. Sifting is preferably performed at a sifter speed range from 500 to 10000 rpm, and / or at a pressure from 0.25 to 5 bar. The skilled person is aware of equipment which can perform the sifting operation, for example, the sifting may be carried out in an air classifier, sifter, or a jet-mill.

The carbon-coated particles of the powder lithium metal phosphate may be prepared by high energy milling of the material obtained from the heating step. Suitably, the high energy milling is carried out in a ball mill having an energy input of less than 1600 kWh/tonne, typically using zirconia milling beads of 0.1 to 1 mm in diameter, typically for a period of 0.5- 20 hours. Alternatively, carbon-coated particles of the powder lithium metal phosphate may be prepared according to the method described in W02005/051840. The process for preparing a composition of the invention typically comprises combining the carbon-coated particles of the agglomerated lithium metal phosphate and the carbon-coated particles of the powder lithium metal phosphate. The combining may take place using a dry mixing process or a wet mixing process.

Dry mixing techniques suitably involve combining the dry particle mixtures in the absence of a liquid. A suitable dry mixing technique uses a plough shear blender or a tumble blender. Typically, the dry mixing is carried out under an inert gas atmosphere.

Wet mixing techniques are well known in the art and typically involve producing a slurry of the carbon-coated particles of the agglomerated lithium metal phosphate and the carbon- coated particles of the powder lithium metal phosphate in a solvent such as, for example, N- methyl-2-pyrrolidone. Suitably, mixers such as a Thinky Mixer, a planetary mixer, or a nonbubble kneader may be used for wet mixing.

The process or use of the present invention may further comprise the step of forming an electrode (typically a cathode) comprising the lithium metal phosphate composition of the invention. Typically, this is carried out by forming a slurry of the lithium metal phosphate composition of the invention, applying the slurry to the surface of a current collector (e.g. an aluminium current collector), and optionally processing (e.g. calendaring) to increase the density of the electrode. The slurry may comprise one or more of a solvent, a binder, carbon material and further additives.

The process or use of the present invention may further comprise constructing a battery or electrochemical cell including the electrode comprising the lithium metal phosphate composition of the invention. The battery or cell typically further comprises an anode and an electrolyte. The battery or cell may typically be a secondary (rechargeable) lithium (e.g. lithium ion) battery.

Electrodes may be formed by any means known in the art. Typically, a slurry of the lithium metal phosphate composition of the invention, a conductive additive and a binder are mixed in a solvent to produce a slurry. The slurry may be coated onto a current collector (e.g. an aluminium sheet) and dried to obtain an electrode. As described above, suitable solvents for producing a slurry include N-methyl-2-pyrrolidone. Suitable binders include Solef Binder 5120. Suitable conductive additives are known to the person skilled in the art. Conductive additives include carbohydrates, such as lactose, maltodextrin, and carbon blacks, such as graphite, or graphene.

Typically a slurry will be prepared comprising the lithium metal phosphate composition of the invention, a conductive additive and a binder in a respective weight ratio of 90:5:5.

The slurry may be applied to a current collector, for example an aluminium sheet, using any method known in the art, to produce a coated current collector. For example, the slurry may be applied to the current collector using a doctor blade.

The coated current collector may be dried in a first drying step to remove volatile components such as the solvent used in produce the slurry. The drying step typically involves heating the coated current collector to 50 °C to produce a partially dried current collector.

The partially dried current collector may be subject to a second drying step to produce the electrode. The second drying step may involve heating the coated current collector to a temperature of from 50 to 250 °C, such as from 100 to 130 °C. A vacuum may optionally be used to assist drying.

Examples

Carbon-coated particles of an agglomerated lithium iron phosphate material were obtained from Johnson Matthey under the trade name P2S2 (herein “agglomerated”, or “agglomerated material”). Carbon-coated particles of powder lithium iron phosphate were obtained from Johnson Matthey under the trade name P2 (herein “powder”, or “powder material”).

Particle Size Distribution

The particle size distributions of the carbon-coated particles of the agglomerated and powder material were analysed using a Malvern MasterSizer 2000.

The particle size distribution for the agglomerated material was measured in air at a pressure of 0.2 bar (as shown in Figure 3), and the particle size distribution of the powder material was measured in ethanol (as shown in Figure 4). The particle size which 25, 50 and 75 volume percent of particles possessed (i.e. 25, 50, or 75 volume percent of particles have a particle size of this value or less) is summarised in the Table 1 below.

Table 1

The peak modes of the agglomerated material, and the peak modes of the powder material are given in Table 2

Table 2

By way of a comparison, the particle size distribution for the agglomerated material was also measured in ethanol (Figure 5), and the particle size distribution of the powder material was also measured in air at a pressure of 0.2 bar (Figure 6).

Preparation of electrodes

Electrodes comprising the agglomerated material and powder material in the weight ratios set out in Table 3 were prepared according to the following general procedure. Example 1 corresponds to a composition according to the invention.

5.4 grams of lithium metal phosphate, comprising agglomerated and I or powder materials in the weight percentages explained in Table 3, was mixed by sequentially adding powder material followed by agglomerated material to 5.4-6.8 grams of N-methyl-2-pyrrolidone in a Thinky Mixer. 3.0 grams of a binder (10 wt% binder in NMP) and 0.3 grams of a conductive carbon (SuperP Li carbon) were added to form a slurry.

Table 3

The particle size distributions of the composition of Example 1 and Comparative Examples 1-4 were measured using a Malvern MasterSizer 2000 in air at a pressure of 0.2 bar, and in ethanol as a carrier solvent. This data is summarised in Table 4 below. Particle size distribution plots for Example 1 in air at a pressure of 0.2 bar, and ethanol, are shown in Figure 1 and Figure 2, respectively.

Table 4

Manufacture of electrochemical cells

A doctor blade was used to coat the electrode compositions on to an aluminium current collector. The target loading was 11-12 mg/cm ² . The electrodes were dried at 120 °C overnight in vacuum.

Electrodes were cut into circular discs (13mm diameter), pressed using a hydraulic press (3 tonnes for 1 minute), dried in an oven over night at 130 °C and transferred into a glovebox.

Cells with coin-cell-type geometry were assembled. Li metal was used as the reference and counter electrode (2-electrode-setup). A glass fibre separator (GF/D Whatman) was used as a separator. Ethylene carbonate:dimethyl carbonate in a weight amount of 1 :1 with 1M LiPFe was used as electrolyte. Electrode Density

Electrode densities were measured by means of a hydraulic press. Electrodes were placed under 3 tonnes pressure for 1 minute and the density of the materials recorded. Electrochemical Testing

Electrochemical cells were tested at a variety of charge/discharge rates (C-rates) from C/10 to 4C, in a voltage range between 2.5 and 4.2 volts.

The direct current resistance (DCR) of electrodes comprising the electrode materials of Example 1 and Comparative Examples 1 to 4 were determined with respect to a current pulse using a Basytec test system. A 1C pulse with a 10 second duration was applied at a 50% state of charge (SOC). Resistances were calculated from the change in voltage and the current from the pulse. R0 was calculated from the values after 1 ms and DCR calculated after 10 seconds.

The electrochemical test results are summarised in Table 5, below.

Table 5 Qualitative assessment of properties of lithium metal phosphate materials

The properties of Example 1 and Comparative Examples 1 to 4 were also assessed on a qualitative basis. The processability based on NMP solvent uptake on slurry preparation, electrode density, gravimetric capacity, volumetric capacity, rate capability, DCR, polarisation and 1 ^st cycle efficiency were graded as “good”, “medium” or “poor”. This assessment is shown in Table 6

Table 6 Results

Table 5 and Table 6 compare the qualitative and quantitative properties of electrode comprising the materials of Example 1 and Comparative Examples 1-4.

Example 1 , according to the present invention, demonstrates an excellent balance of properties.

Example 1 provides an improved electrode density relative to the material which comprises only agglomerated particles with a bimodal particle size distribution (Comparative Example 1), or which include only a small amount of powder (Comparative Example 2). The first cycle efficiency of electrodes comprising the lithium metal phosphates of Example 1 and Comparative Examples 1 to 4 were compared. The first cycle efficiency of electrodes comprising the material of Example 1 gave a surprisingly high first cycle efficiency when compared to those of the Comparative Examples 3 and 4 which contained higher amounts of powder material. The first cycle efficiency of Example 1 was comparable to the Comparative Examples 1 and 2 which contained a greater proportion of agglomerated material.

The gravimetric and volumetric capacity of electrodes comprising the composition of Example 1 indicate particularly good performance across all C-rates. In particular, Example 1 shows improved volumetric capacity versus material formed of purely agglomerated materials having a bimodal particle size distribution (Comparative Example 1). This is surprising as materials having a bimodal particle size distribution have previously been considered to comprise both large (e.g. agglomerated) and small (e.g. primary particles or fragments of agglomerated) and hence would be expected to behave as a mixture when packed/compressed into an electrode. However, the composition of the invention (Example 1) clearly gives improved gravimetric and volumetric capacities.

There was no significant change in RO values for the electrodes prepared from Example 1 and Comparative Examples 1-4. This indicates that the RO values mainly represent the contact resistance to the foil. Electrodes comprising the material of Example 1 show an optimum RO, whereas electrodes comprising greater amounts of powder particles (Comparative Examples 3 and 4) show increased RO values.

Qualitatively, as can be seen from Table 6, electrodes which comprised only agglomerated material with a bimodal particle size distribution (Comparative Example 1) provided generally good properties. However, said materials show poor electrode densities and volumetric capacities. Electrodes comprising only powder materials (Comparative Example 4) show good electrode densities, and medium volumetric capacities, but are poor with respect to the remaining measured parameters.

Sieving versus sifting of agglomerated lithium metal phosphate materials

A sample of agglomerated lithium iron phosphate (P2S2 available from Johnson Matthey) was subjected to either a sieving process or a sifting process to produce a sieved and a sifted material. Sifting was achieved via the use of a jet-mill. The particle size distributions were analysed using a Malvern MasterSizer 2000 using air at a pressure of 0.2 bar. The particle size distribution of the sieved material is shown in Figure 7 and the sifted material in Figure 8.

Figures 7 and 8 show that the sifted material comprises 35 volume percent of smaller particles having a particle size of 3 .m or less, whereas the sieved material comprises 10 volume percent of particles having a particle size of 3 .m or less. That is to say the sifted material comprised a larger volume of small particle size lithium iron phosphate relative to the sieved material. This is believed to be because sieving removes small particle size lithium iron phosphate.

The two materials were formed into electrochemical cells according to the method described above and their electrochemical performance analysed. These results are shown in the Table 7 below.

Table 7

Despite the difference in particle size distribution, and in particular the difference in the volume of smaller particles present (for example, particle size of 3 or less), the electrochemical performance of the sieved and the sifted materials were substantially the same.

This result indicates that the presence of smaller particles (e.g. primary particles or fragments of agglomerates) in agglomerated materials having a bimodal particle size distribution do not have a significant influence on the behaviour of the material when formed into an electrode. This result suggests that the smaller particles present in agglomerated materials with a bimodal particle size distribution do not function in the same way as powder material in the composition of the invention. It further suggests that smaller particles present in agglomerates having a bimodal particle size distribution are not present as “free” particles.

Thus, electrodes comprising only agglomerated material with a bimodal particle size distribution, whether sieved or sifted, in the absence of a powder material, show an inferior balance of electrochemical properties when compared to compositions according to the present invention.